1. Field of the Invention
The present invention relates generally to photolithography for integrated circuits and more specifically to a method of providing levelling and focusing adjustments on a semiconductor wafer and a photolithography apparatus using the method.
2. Description of the Related Art
In a known photolithography process, a chip pattern is printed in each of a plurality of rectangular cells defined on a semiconductor wafer. Since the wafer is of a circular shape, those cells located at or near the periphery of the wafer may be left unexposed to the light of chip pattern. However, if a positive photoresist is applied to the wafer, the unexposed portion of the photoresist survives after the exposed portion is etched away by a solution. For this reason, the peripheral region of the wafer that is coated with a positive photoresist is also exposed to the chip pattern. However, due to the inherent curvature of the wafer near its circumference, the light modulated by the chip pattern is not precisely focused on the peripheral cells, causing some of the peripheral region of the positive photoresist to be left over as undesired remnant material.
According to another photolithography process, several chip patterns are printed on a single mask (or reticle) and the wafer is exposed to all chip patterns of the mask. According to this technique, only those cells where chip patterns are appropriately printed are shipped as marketable products and others are discarded. However, the problem is that the peripheral cells are not exposed to precisely focused light.
Therefore, a need does exist to precisely control the surface of a wafer at all cell locations for appropriate levelling and focusing purposes.
It is therefore an object of the present invention to provide an adjustment method for a semiconductor wafer which precisely align the surface of the wafer at all cell locations and precisely bring the wafer surface to the focal point of impinging light.
According to one aspect of the present invention, there is provided an adjustment method for a semiconductor wafer. The method comprises the steps of (a) setting the wafer in one of a plurality of cell positions, (b) directing a laser beam to the surface of the wafer and detecting light reflecting off the surface of the wafer, (c) analyzing the detected light and determining therefrom a vertical offset position of the wafer and storing offset position data representing the vertical offset position into a first memory, (d) repeating steps (a) to (c) until a plurality of the offset position data are stored in the first memory, determining a plurality of tilt angles of the wafer at the plurality of cell positions from the stored plurality of offset position data, and storing a plurality of angle data representing the determined tilt angles in a second memory, (e) setting the wafer in one of a plurality of cell positions, (f) reading the angle data from the second memory corresponding to the set cell position and horizontally aligning the wafer surface according to the read angle data, and (g) reading the offset data from the first memory corresponding to the set cell position and vertically moving the wafer surface to the focal point according to the read offset position data. Steps (e) to (g) are repeated until the wafer is set to all cell positions.
According to a second aspect, the present invention provides an adjustment method for a semiconductor wafer, comprising the steps of (a) setting the wafer in one of a plurality of cell positions, (b) directing a laser beam to the surface of the wafer at the one cell position and detecting light reflecting off the surface of the wafer, (c) analyzing the detected light and determining therefrom a vertical offset position of the wafer at the one cell position and storing offset position data representing the vertical offset position into a first memory, (d) repeating steps (a) to (c) until a plurality of the offset position data are stored in the first memory, determining, from the stored plurality of offset position data, a plurality of tilt angles of the wafer at a plurality of cell positions close to the periphery of the wafer, and storing a plurality of angle data representing the determined tilt angles in a second memory, (e) setting the wafer in one of a plurality of cell positions, and (f) determining whether or not the set cell location is close to the periphery of the wafer. At step (g), if the set cell location is not close to the periphery, the angle data is read from the second memory corresponding to the set cell position and the wafer surface is horizontally aligned according to the read angle data, and the offset data is read from the first memory corresponding to the set cell position and the wafer surface is vertically moved to the focal point according to the read offset position data. At step (h), if the set cell location is close to the periphery, a plurality of laser beams is directed to the surface of the set cell position of the wafer and beams reflecting off the surface are detected, the detected beams are then analyzed to determine a plurality of vertical offsets of the set cell position and a tilt angle of the wafer at the set cell position is determined, and the wafer surface is horizontally aligned according to the determined tilt angle and the wafer surface is vertically moved to the focal point according to one of the vertical offsets. Steps (e) to (h) are repeated until the wafer is set to all cell positions.
According a third aspect, the present invention provides a photolithography apparatus comprising a mask support system for supporting a mask having a chip pattern and imagewise-modulating incident light with the chip pattern, a projection lens system for projecting the modulated light onto a focal point, a wafer support system for adjustably and horizontally movably supporting a semiconductor wafer, the system being controlled to successively set the wafer in one of a plurality of cell positions, a position sensing system for directing a laser beam to the surface of the wafer at each of the cell positions and detecting light reflecting off the surface of the wafer, position analyzer circuitry for successively analyzing a signal representing the reflecting light of each the cell position and determining therefrom a plurality of vertical offset positions of the wafer at the plurality of cell positions, and angle calculation circuitry for determining a plurality of tilt angles of the wafer at the cell positions from the plurality of vertical offset positions. Control circuitry is provided for controlling the wafer support system to successively set the wafer in one of the cell positions, horizontally aligning, at each cell position, the surface of the wafer according to the tilt angle determined for the cell position and vertically moving, at each cell position, the surface of the wafer to the focal point according to the vertical offset position determined for the cell position.
According to a fourth aspect, the present invention provides a photolithography apparatus comprising a mask support system for horizontally movably supporting a mask having a chip pattern and imagewise-modulating incident light with the chip pattern, a shield member having a slit for allowing a portion of the modulated light to pass therethrough, a projection lens system for projecting the light passing through the slit onto a focal point, a wafer support system for adjustably and horizontally movably supporting a semiconductor wafer, the system being controlled to successively set the wafer in one of a plurality of cell positions, a position sensing system for directing a laser beam to the surface of the wafer at each of the cell positions and detecting light reflecting off the surface of the wafer, position analyzer circuitry for analyzing a signal representing the reflecting light of each the cell position and determining therefrom a plurality of vertical offset positions of the wafer at the plurality of cell positions, and angle calculation circuitry for determining a plurality of tilt angles of the wafer at the cell positions from the plurality of vertical offset positions. Control circuitry controls the wafer support system to successively set the wafer in one of the cell positions, horizontally aligns, at each cell position, the surface of the wafer according to the tilt angle determined for the cell position, vertically moves, at each cell position, the surface of the wafer to the focal point according to the vertical offset position determined for the cell position, and controls the wafer support system and the mask support system to simultaneously move the wafer and the mask in opposite horizontal directions each time the wafer is set in each cell position to linearly scan the set cell position of the wafer with the light focused by the lens system.